Keywords: MDPE, HPPE, creep, material testing, creep law, state surface.

Medium Density Polyethylene (MDPE) and High Performance Polyethylene (HPPE) have become dominant materials for water mains in the United Kingdom water industry [1,2]. Polymer usually exhibits time-dependent behaviour even at ambient temperatures. The service life for the water mains and polymeric linings has to be more than 50 years to make economic sense. Therefore it is important to make reliable life predictions of the pipe-lining using appropriate constitutive models. In order to develop such models it is necessary to establish the stress path and time dependency of the material, and whether the manufacturing and process induces any anisotropy. In addition, in the case of pipe linings, any modification of material properties resulting from the extensive (and largely undetermined) plastic work done by the installation process needs to be assessed. Accordingly the following creep tests to establish the secondary creep path were carried out on both virgin and rolled-down-reverted MDPE pipes [3]. Each test was to be undertaken at a number of stress levels covering the full range of practical interest:

• tension and compression: uniaxial tests on samples cut longitudinally from the pipe wall.
• uniaxial bending: provided shear strains are negligible uniaxial bending behaviour of samples cut longitudinally and circumferentially can be predicted from a combination of the tensile and compressive results. This test then provides a check on the consistency of the three sets of results.
• internal pressure tests: these establish a biaxial stress regime thus providing typical evidence concerning stress path dependency.

In this paper, on the basis of a series of creep tests of MDPE, including tensile creep, longitudinal bending creep and internal pressure creep, the corresponding creep laws and state surfaces were established. In addition, from the tensile creep tests of HPPE the related creep laws were also derived. The established creep laws and state surface are essential to develop theoretical and numerical modelling to predict long-term performance of the pipe lining systems. Using the derived creep law, the behaviour of the longitudinal bending creep and the internal pressure creep of the MDPE pipe were predicted, which was compared with the related test results. Good correlation was obtained.

1

J.C. Boot, Z.W. Guan and I. Toropova, "The structural performance of thin-walled polyethylene pipe linings for the renovation of water mains", Tunnelling and Underground Space Technology, Volume 11, Supplement 1, Pages 37-51, 1996. doi:10.1016/0886-7798(95)00038-0

2

J.C. Boot and I.L. Toropova, "Polyethylene thin-walled linings for water mains: development of structural design guidelines", Tunnelling and Underground Space Technology, Volume 14, Supplement 2, Pages 13-28, 1999. doi:10.1016/S0886-7798(00)00022-5

3

Z.W. Guan, The structural behaviour of thin-walled polymeric pipe linings, Ph.D. Thesis, University of Bradford, 1993.

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